The present invention relates to a rotary lawn mower of the type which includes a shroud supported on wheels for traversing the surface of a lawn, a blade assembly which is horizontally rotatable within the shroud, a shaft extending vertically downward at the center of the shroud and attached to the blade assembly for driving it in horizontal rotation, and an opening in the shroud for the exit of cuttings from the lawn.
The cutting of grass, if a science at all, is purely an experimental one. Some kind of sharp instrument must be applied to a stalk of grass if it is to be cut. In early times the scythe--a swinging blade--was used to cut grass or other vegetation. About 1830 Budding invented what is now known as the reel type mower. It uses a fixed blade moving horizontally against the grass, and rotating cutters arranged in a cage-like configuration which mesh with the upper surface of the fixed blade once each revolution. In that type of machine the stalks of grass are grasped between a moving cutter and the stationary blade.
A more recent development is the rotary mower with a horizontally rotating blade assembly. The blade assembly is motor driven, rotates at high speed, and relies on the stiffness of the stalk of grass to complete the desired cutting action. In U.S. Pat. No. 1,656,105 issued in 1928 Durkee disclosed the use of air lifts on the rotating blade arms of a rotary mower. The patent explains that such air lifts "caused the growth to be straightened up, and in fact drawn into the paths of rapidly moving cutting edges". As is now well known, the air lifts when used in conjunction with a shroud enclosing the blade assembly also create a vacuum that aids in lifting the cuttings and expelling them through an exit port provided for that purpose.
A type of rotary blade member now in widespread usage includes a single piece of metal which is of essentially constant width throughout its length, has a central hole for attaching the drive shaft of the motor, and each of its ends provides a cutting arm which has one side sharpened while its other side is inclined upwardly to provide an air lift. A central portion of the blade member is dull on both of its sides. The rotating cutting edges are potentially dangerous to human beings, and when concealed by the shroud their operation cannot be fully observed. In a scientific sense, therefore, the precise mechanism of the cutting action is not fully understood. There are, however, well-known problems or deficiencies in the operation of these machines. The blade member may experience a heavy frictional drag which then results in stalling the motor. Cuttings may be left on the ground behind the machine, presumably because the air lifts have created an inadequate vacuum. And a further problem is the "scalping" of the grass.
Scalping occurs when stalks of grass are knocked over and damaged, but are not cut. It may then become necessary to run the mower over the same piece of ground a second time to complete the job, and even then some uncut grass may remain. Also, the scalped grass generally creates an increased frictional drag on the machine, slowing its operation.
More specifically, when a particular cutting arm of the rotating blade member is at the forward edge of the machine near the front wheels, it is the cutting edge of that cutting arm that directly engages the grass. But the action of the blade may result in "scalping" some stocks of the grass. The cutting edge then moves over the particular stalks, damaging them and bending them over, but not cutting them. Any stalks of the grass which are not cut by the rotation of a cutting arm at the forward edge of the machine may later be engaged near the rearward portion of the machine. Presumably the previously uncut stalks are now bent toward the cutting arm whose cutting edge will be able to engage them from their under sides. However, in actual practice the previously scalped stalks may not rise far enough above the level of the blade to be cut, and hence will become even more badly bent and remain uncut.